Microwave powered linear accelerators are in widespread use for radiotherapy treatment, radiation processing of materials and physics research. In general, such accelerators include a charged particle source such as an electron source, an accelerator guide that is energized by microwave energy and a beam transport system.
In many applications of these accelerators, it is desirable to be able to adjust the final energy of the accelerated particles. For example, the linear accelerator may be used to treat a variety of cancers by delivering a high local dose of radiation to a tumor. Low energy beams may be used to treat certain types of cancers, while higher energy beams may be desirable for deep seated tumors. In general, it is desirable to provide radiation treatment systems that generate beams having energies that can be tailored to the patient's tumor.
Although linear accelerators operate optimally at one energy level, a variety of techniques have been used for varying the output energy of linear accelerators. One approach is to vary the microwave input power to the accelerator guide. This approach has the disadvantages of increasing the energy spread of the beam, reducing electron beam capture and having a limited adjustment range. Another approach has been to use two accelerator guide sections. The microwave power supplied to the accelerator guide sections is variable in amplitude and phase. The particles may be accelerated or decelerated in the second accelerator guide section. An attenuator and a phase shifter are used to control output energy. Such systems tend to be large, complex and expensive.
Other prior art configurations for producing variable energy outputs have included systems in which the beam passes through the accelerator guide two or more times. An example of such a system is the microtron in which electrons make multiple passes of increasing radius through a microwave cavity, and an orbit having the desired energy is selected as the output. Yet another approach uses an energy switch in a side cavity on the accelerator guide.
Prior approaches to variable energy linear accelerators are described by C. J. Karzmark in "Advances in Linear Accelerator Design for Radiotherapy", Medical Physics, Vol. 11, No. 2, March-April, 1984, pages 105-128 and by J. A. Purdy et al in "Dual Energy X-Ray Beam Accelerators in Radiation Therapy: An Overview", Nuclear Instruments and Methods in Physics Research, B10/11, 1985, pages 1090-1095. Variable energy linear accelerators are also disclosed in U.S. Pat. No. 4,118,652, issued Oct. 3, 1978 to Vaguine and U.S. Pat. No. 4,162,423 issued Jul. 24, 1979 to Tran.
All of the prior art approaches to varying the energy level of a linear accelerator have had one or more disadvantages, including a failure to maintain a narrow energy spectrum at different output energy levels, difficulties in adjusting the energy level, a high degree of complexity, high cost and large physical size.